1. Field of the Invention
This invention relates to an engine that utilizes air moving at supersonic speeds for compression, combustion and expansion. Such an engine is known as a scramjet. More particularly, a pilot pod is centrally disposed within an isolator of a scramjet module. As a result, the hottest combustion gases are located within the core of the combustor, rather than along the walls, thereby reducing combustor heat load.
2. Description of the Related Art
Engines that use ram compression instead of a mechanical compressor to pressurize air for combustion and expansion are known as ramjets. When the flowpath through the engine is designed specifically for higher speeds where supersonic combustion is superior, that is typically at speeds above Mach 5 or 6, the engine is referred to as a scramjet. A simplified version of a prior art scramjet is illustrated in FIG. 1. The two dimensional scramjet 10 is generally symmetric about axis 12 and includes a main isolator 14 that connects the scramjet intake 16 to a main combustor 18. The main isolator 14 permits raising the air pressure higher than the scramjet intake 16 can generate at a given flight speed and altitude. While not required for a scramjet, the main isolator 14 is vital for dual mode ramjets that are capable of operation in both subsonic and supersonic environments. A suitable fuel 19 is introduced into the airflow through fuel injectors 20. Combustion of the fuel/air mix generates very high temperatures and rapid expansion of gaseous combustion products. Expulsion of these combustion products through a nozzle (not shown) downstream 22 of the main combustor 18 generates thrust.
The high velocity and low pressure flow of air and fuel, within the main combustor 18 makes it difficult to sustain combustion. In most scramjet engines the combustion will only take place when a suitable pilot zone ignites and an incoming fuel/air mixture and then propagates across the duct with a turbulent flame front. This flame travels normal to the air at a fraction of the mean air velocity so the flame front appears to be swept back at a large angle. In prior art scramjets, with the pilot 24 situated on the main combustor wall 26 of the scramjet 10 that defines the exterior wall of the main combustor 18, the exterior wall 26 is immediately exposed to full combustion temperature while the flame slowly moves radially inward to burn the rest of the air. Dotted isotherm lines 28 illustrate a demarcation between hottest region 30 (e.g. typical total temperature in excess of 6000° R), moderate region 32 (e.g. typical total temperature between 5000° R and 6000° R), and coolest region 34 (e.g. typical temperature less than 5000° R). The hottest region 30 generates a high combustor heat load on the exterior walls 26 of the main combustor 18 which are exposed to the most severe thermal environment. As a result, the walls must be made from exotic high temperature resistant materials such as tungsten or actively cooled with scarce fuel increasing costs and complexity.
U.S. Pat. No. 4,170,110 to Radin discloses a scramjet where the intake air is divided into a central stream and peripheral boundary layer streams. The peripheral boundary layer streams are very narrow, on the order of 15 microns in thickness. A typical scramjet with a non-axisymmetric, two dimensional (2-D), pilot is illustrated in U.S. Pat. No. 5,253,474 to Correa, et al. Both U.S. Pat. No. 4,170,110 and U.S. Pat. No. 5,253,474 are incorporated by reference herein in their entireties.
There remains a need for a scramjet having a reduced heat load applied to the wall of the combustor as compared to the prior art.